Swift equivalent of Java toString()

Question

What is the Swift equivalent of Java toString() to print the state of a class instance?

Answer 1

The description property is what you are looking for. This is the property that is accessed when you print a variable containing an object.

You can add description to your own classes by adopting the protocol CustomStringConvertible and then implementing the description property.

class MyClass: CustomStringConvertible {
    var val = 17

    public var description: String { return "MyClass: \(val)" }
}

let myobj = MyClass()
myobj.val = 12
print(myobj)  // "MyClass: 12"

description is also used when you call the String constructor:

let str = String(myobj)  // str == "MyClass: 12"

This is the recommended method for accessing the instance description (as opposed to myobj.description which will not work if a class doesn't implement CustomStringConvertible)

Answer 2

How it's done with NSObject extended classes

If your model class is extended from NSObject, you have to override the Variable description as follows:

public override var description: String {
    return "\n{\n index: \(self.index),\n"
        + " country: \(self.name),\n"
        + " isoCountryCode: \(self.isoCountryCode),\n"
        + " localeId: \(self.localeId),\n"
        + " flagImageName: \(self.flagImageName!)\n}"
}

You can check how I have done it here within the Country class, in the "CountryPicker iOS Swift library".

Or, to make it simpler for you to understand, your class and description method should look like following:

public class MyClass: NSObject {
   public var memberAttribute = "I'm an attribute"

   public override var description: String {
       return "My Class member: \(self.memberAttribute)"
   }
}

Note: Since you are extending your Modal class from NSObject it doesn't require for your class to comply with CustomStringConvertible class anymore, and you are overriding description variable from the NSObject class itself. Always remember, CustomStringConvertible is mostly the pure Swift way of achieving this.

Answer 3

If it is possible to use the struct instead of class, then nothing additional to do.

struct just prints fine itself to the output

print("\(yourStructInstance)")

or with class like this:

print(String(describing: yourClassInstance))

Answer 4

You should use String(obj).

Direct from the documentation for CustomStringConvertible:

NOTE

String(instance) will work for an instance of any type, returning its description if the instance happens to be CustomStringConvertible. Using CustomStringConvertible as a generic constraint, or accessing a conforming type's description directly, is therefore discouraged.